Extraction, identification and mechanism of action of antibacterial substances from Galla chinensis against Vibrio harveyi

The aim of this study was to investigate the extraction of antibacterial substances from Galla chinensis (ASGC) that act against the aquatic pathogen Vibrio harveyi, to identify the chemical composition and explore the mechanism of action. The optimal extraction process was as follows: anhydrous methanol, liquid/material ratio (mL/g) 26:1, extraction temperature 98 °C and extraction time 157 min. The diameter of the inhibition zones (21.31 ± 0.51 mm) under the optimal conditions agreed with the predicted value by experimental rechecking. The yield of ASGC was 39.39% ± 0.69%, and two compounds including gallic acid (GA) and penta-O-galloyl-β-d-glucose (5GG) were identified in ASGC. The minimal inhibitory concentration (MIC) values of ASGC and GA were 0.50 and 0.25 mg/mL, respectively. Meanwhile, 5GG and other chemical constituents did not exert antibacterial activities against V. harveyi (MICs > 1 mg/mL). Further investigation of the mechanisms of action of ASGC and GA against V. harveyi revealed their strong destructive effects on the growth of bacterial cells, the integrity and permeability of the cell membrane, and morphological alterations. The report provides a favorable prospect of GA as a potential control agent against V. harveyi in aquaculture.